Such an oscillator element is also used in the photo acoustic trace gas detector described in the United States patent application, published as US 2005/0117155. The photo acoustic trace gas detector uses a laser beam for exciting molecules of a trace gas in a gas mixture. The excitation of the trace gas molecules results in an increased temperature and pressure. The laser beam is wavelength or amplitude modulated for generating pressure variations in the gas mixture. In the photo acoustic trace gas detector according to US 2005/0117155, a quartz tuning fork detects the pressure variations. After amplification by a pre-amplifier, a lock-in amplifier mixes the tuning fork signal with a reference signal for acquiring an output signal. The use of the quartz tuning fork for the detection of the pressure variations allows for a relatively compact photo acoustic trace gas detector.
An important application of photo acoustic trace gas detectors is breath testing. Breath testing is a promising area of medical technology. Breath tests are non-invasive, user friendly and low cost. Prime examples of breath testing are monitoring of asthma, alcohol breath testing and detection of stomach disorders and acute organ rejection. First clinical trials show possible applications in the pre-screening of breast and lung cancer. These volatile biomarkers have typical concentrations in the parts per billion (ppb) range. Nitric oxide (NO) is one of the most important trace gases in the human breath, and elevated concentrations of NO can be found in asthmatic patients. Currently, exhaled NO levels at ppb concentrations can only be measured using expensive and bulky equipment based on chemiluminescence or optical absorption spectroscopy. A compact, low-cost NO sensor forms an interesting device that can be used to diagnose and monitor airway inflammation and can be used at the doctor's office and for medication control at home.
For detecting NO in the exhaled breath, an approach has been chosen where NO is chemically converted into NO2 which is subsequently detected with a photo acoustic sensor incorporating a blue semiconductor laser. NO2 has a broad absorption spectrum in the blue wavelength range, and consequently wavelength modulation is not the preferred modulation method for detecting NO2. Unfortunately, amplitude modulation of the laser power leads to large background signals that easily dominate the small NO2 related photo acoustic signals. It is a problem of the photo acoustic trace gas detector according to US 2005/0117155 that the accuracy of the measurements is not sufficient.